US7285099B1 - Bias-probe rotation test of vestibular function - Google Patents
Bias-probe rotation test of vestibular function Download PDFInfo
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- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/48—Other medical applications
- A61B5/4863—Measuring or inducing nystagmus
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
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- A61B5/4023—Evaluating sense of balance
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/40—Detecting, measuring or recording for evaluating the nervous system
- A61B5/4029—Detecting, measuring or recording for evaluating the nervous system for evaluating the peripheral nervous systems
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Definitions
- This invention relates generally to quantitative assessment of vestibular function, more particularly to methods and apparatus for medical evaluation of patients with balance and equilibrium complaints.
- the medical evaluation of patients with balance and equilibrium complaints often includes an assessment of their vestibular function.
- the goal of this assessment is to determine if peripheral vestibular function is normal or abnormal. If vestibular function is found to be abnormal, is one ear involved or are both ears affected? How severe is the abnormality? Is the abnormality stable or fluctuating? Standard clinical tests frequently do not provide adequate answers to one or all of these questions.
- Conventional clinical rotation testing typically uses sinusoidal or velocity step motion with moderate stimulus amplitudes (50-100°/s peak velocity) to assess vestibular function by providing a natural rotational stimulus to the semicircular canals and measuring eye movements evoked by the vestibulo-ocular reflex (VOR).
- Conventional rotation testing has good test-retest reliability, but relatively low sensitivity. Testing sometimes fails to detect a vestibular abnormality or, if an abnormality is detected, to provide a detailed assessment of the severity and/or the side of lesion.
- VOR reflexive eye movements
- the VOR reflexive eye movements
- Visual acuity is maintained during head movement by the generation of compensatory eye movements that maintain the direction of gaze fixed in space.
- the VOR, smooth pursuit, and optokinetic reflex systems work together to generate these compensatory eye movements.
- compensatory eye movements are generated only by the VOR. Therefore, measurements of VOR eye movements in the dark (or sometimes using high frequency head motions where visual reflexes are ineffective) provide an indirect means of assessing peripheral vestibular function.
- Rotation testing includes both conventional passive rotations (sometimes referred to as slow harmonic acceleration or SHA testing) and active, subject initiated autorotation tests. More recently, the “head impulse test” or “Halmagyi head thrust” test has become popular as an easily applied qualitative method for detecting the existence of bilateral or unilateral vestibular dysfunction.
- Caloric test artificially stimulates the inner ear vestibular receptors using either warm-water or cold-water irrigations of the external ear canals. This evokes eye movements, which are measured and compared across ears to determine vestibular asymmetry.
- Patients are placed in a supine position with the head elevated about 30° in a darkened room. This head position places one set of vestibular receptors, the horizontal semicircular canals, into an earth-vertical orientation.
- An irrigation creates a thermal gradient across the inner ear that stimulates the horizontal canal, primarily by inducing a convective fluid movement within the distal loop of the canal, and secondarily, by direct thermal effects.
- This fluid movement stimulates receptor hair cells, which in turn modulate the activity of the 8 th nerve afferents that innervate the horizontal canal.
- a warm water irrigation results in an increased afferent discharge rate, and a cold water irrigation causes a decreased discharge rate.
- the increased discharge caused by warm irrigations evokes a sensation of sustained rotation toward the irrigated ear and evokes a compensatory VOR eye rotation away from the irrigated ear.
- Cold water irrigations evoke oppositely directed sensations and compensatory eye movements.
- “Slow phase” compensatory eye movements are interspersed with “fast phase” eye movements that reset the eye position towards the straight ahead gaze position, producing a triangular-shaped eye position waveform referred to as vestibular-evoked “nystagmus.”
- the slow phase and fast phase components are separated from one another.
- the slow phase component is analyzed by calculating its slope which gives the slow phase eye velocity (units °/s) for each beat of nystagmus.
- the peak velocity is taken as a measure of the responsiveness of the ear to a particular irrigation.
- a complete caloric test typically consists of measuring the peak velocity response to four separate irrigations (both warm and cold irrigations in each of the two ears). These four peak velocity measures are scored by the calculation of Jongkees' percentage measures of “reduced vestibular response” (RVR), sometimes referred to as canal paresis, and “directional preponderance” (DP) (See reference 35 listed below). If responses are significantly different in the two ears (typically RVR greater than 25% difference), the ear with the lower response is typically considered to be abnormal. If all four irrigations produce below normal or absent responses, this implies that the patient may have bilaterally reduced or absent vestibular function.
- RVR reduced vestibular response
- DP directional preponderance
- the DP measure compares the responses of irrigations that produce right-beating nystagmus with those that produce left-beating nystagmus. If DP is abnormal (typically greater than 25%), this suggests that some non-specific uncompensated imbalance of vestibular function is present.
- the chief advantage of the caloric test is that each ear is stimulated individually. This allows for the identification of reduced vestibular function in an ear even though the patient might be well compensated for the lesion and may not express any other overt signs of an acute vestibular lesion.
- the RVR and DP measures are quantitative in nature and can be used to grade the severity of the vestibular asymmetry.
- Caloric testing has several significant limitations.
- the thermal stimulus that reaches the inner ear depends upon many anatomical factors (i.e., temporal bone thickness, dimensions of middle ear space, fluid in the middle ear space, variation in blood flow) and procedural factors such as the technician's skill.
- anatomical factors i.e., temporal bone thickness, dimensions of middle ear space, fluid in the middle ear space, variation in blood flow
- procedural factors such as the technician's skill.
- there is high variability across subjects in delivery of the thermal stimulus to the inner ear due to differences in temporal bone thickness dimensions of middle ear space, fluid in the middle ear space, and variations in blood flow. These factors make it difficult to detect small differences in responses between both ears.
- test-retest reliability is poor, making the test a poor choice (unsuitable) for tracking changes in vestibular function over time.
- response variability limits the detection of small differences in responses between the two ears.
- the conventional rotation test involves a patient being rotated upright in a clinical rotation chair in a completely dark room.
- the chair is rotated about an earth-vertical axis, with rotations of moderate amplitude (50-100°/s peak velocity).
- the typical rotations are sinusoidal, with frequencies ranging from 0.01 to 1.0 Hz.
- Rotational velocity step stimuli and sometimes pseudorandom or sum-of-sines stimuli are also used.
- Rotation testing differs from caloric testing in that a natural rotational stimulus, which stimulates both ears simultaneously, is used to evoke compensatory VOR eye movements.
- Patients are tested in a completely dark room to eliminate visually generated eye movements. During testing, they are seated upright in a chair mounted on a servo-controlled motor. The motor delivers accurately controlled rotational motions of the chair about an earth-vertical axis. This motion stimulates primarily the horizontal semicircular canals in both ears.
- a rotation towards the right causes an increased neural discharge rate in 8 th nerve afferents innervating the right side horizontal canal, and a decreased discharge rate in afferents innervating the left horizontal canal. The opposite occurs for rotations to the left.
- the central nervous system uses this “push-pull” neural activity in the two ears to generate VOR eye movements in a direction opposite to the head rotation.
- Rotation-evoked nystagmus is analyzed in a manner similar to caloric-evoked nystagmus. The nystagmus is separated into slow and fast phase components. The slope of the compensatory slow phase component provides a measure of the slow phase eye velocity over time. With sinusoidal stimulation, this slow phase eye velocity component is sinusoidally modulated at the stimulus frequency.
- VOR gain response amplitude divided by stimulus amplitude
- VOR phase timing of the response relative to the stimulus
- VOR bias average value of slow phase eye velocity over a complete stimulus cycle
- VOR gain asymmetry comparison of VOR gain during rotation to the right versus rotation to the left.
- the natural stimulus used by rotation testing provides several advantages over caloric testing.
- the test-retest reliability of rotation testing is good, making rotation testing amenable to tracking function over time.
- the rotation test VOR gain measure has a limited range of normal values, making rotation testing particularly useful in assessing bilateral loss of vestibular function.
- the repetitive nature of the stimulus affords great opportunity to use averaging to improve test reliability and the possibility to obtain useful results from partially corrupted data records.
- rotation testing is well tolerated by nearly all patients and only rarely evokes nausea.
- the CNS is able to compensate for the acute effects of a unilateral loss of vestibular function which might otherwise be used to identify the existence of a unilateral vestibular deficit.
- an acute loss of vestibular function results in a strong “spontaneous nystagmus.”
- This nystagmus occurs because the CNS normally compares the neural activity in the two ears and generates compensatory eye movements proportional to the difference in activity between the two ears.
- This spontaneous nystagmus typically diminishes over a time course of several days as the CNS rebalances the central VOR neural mechanisms.
- a second important limitation in rotation testing is its poor sensitivity in detecting a compensated partial unilateral vestibular loss (inability to reliably identify any abnormality in patients who have only a partial loss of vestibular function).
- a recent study demonstrated that VOR gain measures from rotation tests using both velocity step and sum-of-sines stimuli were uncorrelated with the severity of unilateral vestibular dysfunction as characterized by the caloric RVR measure (See reference 57 listed below).
- a third limitation of conventional rotation testing is the ambiguity between rotation test results in patients with a partial bilateral loss and a compensated unilateral loss (inability to distinguish between patients with a partial bilateral vestibular loss and a compensated unilateral vestibular loss of function). Both abnormalities produce a reduction in the VOR time constant and, equivalently, a low frequency phase advance.
- a partial bilateral vestibular loss may cause only a mild reduction in VOR gain such that the VOR gain may remain within the normal range while the VOR time constant is reduced. This pattern of normal gain and reduced time constant is indistinguishable from a compensated unilateral vestibular loss pattern.
- the autorotation test evaluates the VOR at higher frequencies of head rotation (2-6 Hz) than those in conventional rotation tests. Testing has become standardized using commercially available systems. Tests are performed in the light with the subject instructed to gaze at a fixed visual target. An audio tone cues the test subject to oscillate his/her head in time with the tone. The tone cue begins at 0.5 Hz and continuously increases to 6 Hz in about 20 s. Head movements at lower frequencies are used to calibrate the eye movement recordings, and higher frequency VOR responses are quantified using spectral analysis techniques to calculate response gain and phase at frequencies of 2 to 6 Hz. Deviations of gain and phase responses from normal ranges are indicative of abnormal vestibular function.
- Head impulse test In the head impulse test, an examiner rotates a patient's head with a rapid, high acceleration rotation though an angle of about 20-30° while the patient attempts to maintain higher gaze on a fixed target. The examiner looks for corrective eye movements following the head rotation, indicating that vestibular function is deficient and unable to generate VOR eye movements that fully compensate for the head rotation. Typically, the rotation is about the head's vertical axis which stimulates primarily the horizontal canals. The patient attempts to maintain his/her gaze fixed on a target during this maneuver. In patients with severe canal paresis, the rotation towards the dysfunctional ear or ears produces an inadequate compensatory VOR.
- This inadequate VOR causes the eyes to move off target, with the result being that a visually guided corrective saccade is generated to reacquire the target at the conclusion of the head rotation.
- the presence of this corrective saccade is a convenient qualitative clinical sign indicating abnormal canal function.
- the gain of the VOR can be calculated and used as an indicator of VOR function.
- the main advantage of head impulse testing is that the qualitative version of the test (i.e. using the presence of a corrective saccade as a sign of abnormality) can be performed by a knowledgeable practitioner with no equipment. In cases where patients were known to have a complete unilateral loss of vestibular function, the test has been shown to have 100% sensitivity and specificity. In addition, recent research indicates that this technique can be extended to include head rotations about oblique axes that allow for evaluation of the vertical semicircular canals.
- head impulse testing The chief disadvantage of head impulse testing is that it has recently been shown in a study to have poor sensitivity in cases of less severe canal paresis (See reference 7 listed below). This blinded study compared the results of conventional caloric testing with head impulse testing. In patients with severe canal paresis (75-100% RVR on caloric testing), head impulse testing showed abnormal results in 77% of these patients. However, head impulse testing revealed abnormalities in only 9.5% of patients with moderate paresis (50-75% RVR) and 0% of patients with mild paresis (25-50% RVR). Overall, it was concluded that head impulse testing was useful in detecting severe paresis, but could not serve as a replacement for the caloric test. Major limitations are that: (1) it doe not adequately identify mild-to-moderate vestibular dysfunction in a single ear, and (2) the test cannot be used in patients with any limitations in neck mobility (e.g. limitations due to whiplash, arthritis).
- patients with neck injury or other limitations in neck mobility cannot be tested using rapid head on body rotations.
- Patients in this category would include patients with balance complaints associated with accidents causing whiplash injuries, and elderly patients with limited mobility due to arthritic conditions.
- FIG. 1 depicts a block diagram of an embodiment for a system having a device to rotate a subject to be tested and a motion control to control the motion of the device, in accordance with the teachings of the present invention.
- FIG. 2 illustrates an embodiment for a two-sine rotational stimulus, in accordance with the teachings of the present invention.
- FIG. 3A illustrates an embodiment of an acceleration pulse, step, and sinusoidal for a pulse-step-sinusoidal (PSS) rotational stimulus, in accordance with the teachings of the present invention.
- PSS pulse-step-sinusoidal
- FIG. 3B illustrates an embodiment of a velocity waveform for a pulse-step-sinusoidal rotational stimulus, in accordance with the teachings of the present invention.
- FIG. 4A illustrates an embodiment of a 2-sine stimulus waveform, in accordance with the teachings of the present invention.
- FIG. 4B illustrates an embodiment of a schematic response the 2-sine stimulus waveform of FIG. 4A in a normal subject, in accordance with the teachings of the present invention.
- FIG. 4C illustrates an embodiment of a schematic response the 2-sine stimulus waveform of FIG. 4A in a right side unilateral loss subject, in accordance with the teachings of the present invention.
- FIG. 5 shows a block diagram used to simulate VOR responses of an embodiment for testing, in accordance with the teachings of the present invention.
- FIGS. 6A-6F show the simulated VOR output (simulated VOR slow phase eye velocity response to a 2-sine stimulus in normal and left unilateral loss subjects) for the simulation of FIG. 5 , in accordance with the teachings of the present invention.
- FIGS. 7A-7E show the portion of the simulated VOR eye velocity related to the high frequency probe component of a 2-sine rotational stimulus for the embodiment of the simulation of FIG. 5 , in accordance with the teachings of the present invention.
- FIGS. 8A-8F show a VOR slow phase horizontal eye velocity data recorded from one representative normal subject and four unilateral vestibular loss subjects in an embodiment using a 2-sine stimulus, in accordance with the teachings of the present invention.
- FIGS. 9A-9F show a modulation of a VOR response to the probe component with the VOR slow phase velocity data filtered using a 0.5 to 5 Hz bandpass filter from the one representative normal subject and the four unilateral vestibular loss subjects of
- FIGS. 8A-8F in an embodiment using a 2-sine stimulus, in accordance with the teachings of the present invention.
- FIG. 10A depicts a block diagram of an embodiment for analysis of a probe component, in accordance with the teachings of the present invention.
- FIG. 10B illustrates data from a subject of FIGS. 9A-9F corresponding to the block diagram of FIG. 10A , in accordance with the teachings of the present invention.
- FIG. 11 shows a plot of the modulation factor, m, for the three normal and four unilateral loss subjects of the experiment related to FIGS. 8A-8F and FIGS. 9A-9F as a function of the bias component stimulus amplitude, in accordance with the teachings of the present invention.
- FIG. 12 depicts a block diagram of an embodiment of a method to analyze the bias components of the 2-sine stimulus, in accordance with the teachings of the present invention.
- FIG. 13 shows an embodiment for a input-output relationship generated according to the embodiment in FIG. 12 , in accordance with the teachings of the present invention.
- FIG. 14 shows the results of an embodiment of the input-output analysis of FIG. 12 for the four unilateral loss subjects and two normal subjects of FIGS. 8A-8F , in accordance with the teachings of the present invention.
- FIGS. 15A-15D show the results of simulation analyses using embodiments for the probe analysis and input-output analysis procedures of FIGS. 10A and 12 , in accordance with the teachings of the present invention.
- FIGS. 16A-16C show embodiments of the stimulus acceleration, velocity, and simulated semicircular canal afferent discharge rate profiles for a PSS stimulus, in accordance with the teachings of the present invention.
- FIG. 17A shows an embodiment for step component measures for a test using PSS rotational stimulus, in accordance with the teachings of the present invention.
- FIG. 17B shows an embodiment for sine component measures for a test using PSS rotational stimulus, in accordance with the teachings of the present invention.
- FIGS. 18A-18B show eye movement data in normal subjects measured in the dark and measured with a light fixation for an embodiment of a PSS rotational stimulus, in accordance with the teachings of the present invention.
- FIGS. 19A-19B show eye movement data in subjects with unilateral vestibular loss measured in the dark and measured with a light fixation for an embodiment of a PSS rotational stimulus, in accordance with the teachings of the present invention.
- FIGS. 20A-20B show response parameters obtained from a fixation test performed on subjects with normal vestibular function and patients with unilateral vestibular loss for an embodiment of a PSS rotational stimulus, in accordance with the teachings of the present invention.
- methods and apparatus for new rotation test stimulus and analysis methods overcome many of the limitations of conventional clinical tests of peripheral vestibular function, such as the four clinical tests described above; These embodiments provide a set of clinical tests that aid in the diagnosis of patients exhibiting symptoms of dizziness and balance instability, by determining whether inner ear (vestibular) asymmetries exist and, if so, identifying which ear is involved and the severity of the asymmetry.
- Embodiments of these methods and apparatus provide for new testing that delivers precise, repeatable stimuli to the vestibular system, can identify the existence of asymmetric vestibular function even in well compensated subjects, can identify which ear is dysfunctional, is well tolerated by most test subjects, can be performed in patients with limited neck mobility, and does not require active participation by the test subject.
- Embodiments include a set of test stimuli and associated analysis methods that can be used to characterize an asymmetry of inner ear (vestibular) balance function.
- Various embodiments are based on an underlying principle that vestibular responses in one ear can be turned off, to allow responses in the other ear to be evaluated. In an embodiment, this is accomplished using a novel 2-component stimulus that is designed to control the motion of a conventional clinical rotation chair (a device that rotates a seated patient about a vertical axis).
- FIG. 1 depicts a block diagram of an embodiment for a system 100 having a device 110 to rotate a subject to be tested and a motion control 120 to control the motion of the device 110 .
- stimuli are provided to device 110 to turn off vestibular responses in one ear of the subject, while allowing vestibular responses in the other ear to be evaluated.
- device 110 is a clinical rotation chair.
- device 110 and motion control 120 are integrated.
- system 100 is a clinical rotation chair.
- rotation of device is about an earth-vertical axis with a subject's head oriented 20° nose down from Reid's plane to place the horizontal canal plane perpendicular to the axis of rotation.
- Medical centers and clinics that specialize in diagnosing balance disorders usually have conventional clinical rotation chair systems. Several medical equipment companies manufacture these systems.
- Embodiments for the rotation test and analysis may be incorporated into these existing systems by modifying the software that controls the movement of the chair in which the patient, subject, is seated and analyzes the evoked eye movements. In most cases, modifications to the existing rotation chair equipment (motors, eye movement recording equipment) would be minimal.
- the novel rotation test and analysis may be incorporated into the clinical rotation chair systems at the time of manufacture, at a nominal cost, since the expense would not involve any new equipment but rather the novel programming of the rotation chair systems to include the new stimulus and analysis.
- tests are performed with the subject seated either in a completely dark room, or in an otherwise dark room except for a small illumined visual target on which the subject visually fixates and that rotates with the subject.
- rotational motion stimulates the vestibular motion sensors in the inner ear, and information from these motion sensors is used by the central nervous system to generate eye movements though a reflex known as the vestibulo-ocular reflex (VOR).
- VOR vestibulo-ocular reflex
- visual information is used by the central nervous system to partially suppress the VOR eye movements evoked by the rotational stimulus.
- eye movements are recorded and analyze, to obtain measures that characterize the symmetry of responses to rightward- and leftward-directed rotational motions. A significant asymmetry of these responses indicates that vestibular function is not equal in the two ears, and they also provide information about which ear is deficient and the extent of the deficiency.
- An embodiment includes a first type of rotational stimuli applied to the rotational motion for testing.
- Another embodiment includes a second type of rotational stimuli applied to the rotational motion for testing.
- Each type concludes two separate components referred to as the “bias” and “probe” components.
- the bias component waveform has a high-amplitude and long duration rotational motion, while the probe component has a low-amplitude and high-frequency motion.
- the bias component for rotational motion is designed to temporarily turn off vestibular responses in one ear while the responsiveness in the opposite ear is simultaneously evaluated using the probe component of the stimulus. For example, rotational motion toward the right excites activity in the horizontal semicircular canal of the right ear (the vestibular motion sensor most affected by rotation when the head is in an upright position).
- This rotational motion toward the right inhibits activity in the left ear's horizontal semicircular canal.
- the bias component amplitude is designed to be large enough to completely inhibit the activity in the left horizontal canal during a portion of the rightward motion of the chair.
- the left side motion receptor is unable to encode information related to the probe component motion.
- the probe component motion is encoded by this ear and VOR eye movements related to the probe component are generated.
- right-side horizontal canal function is absent, then neither ear is able to encode the probe component motion, and no VOR eye movements related to the probe component are generated.
- the absence of VOR eye movements related to the probe component is indicative of the absence of right-side vestibular function.
- the first rotational stimulus is designed to operate with lower torque motors commonly used in conventional clinical rotation test systems.
- the first type of rotational stimulus is a two-sine stimulus.
- This first type of rotational stimulus includes two sinusoidal components, as shown in FIG. 2 .
- the bias component, or bias is provided by a low frequency and high amplitude sinusoidal waveform relative to the probe component, or probe, which is added to the bias having a higher frequency and lower amplitude sinusoidal waveform than the bias.
- the bias component, or bias is provided by a low frequency (0.1 Hz or less) and high amplitude (150-250°/s peak velocity) sinusoidal waveform.
- the probe is added to the bias and includes a higher frequency (about 1 Hz) and lower amplitude (10-20°/s peak velocity) sinusoidal waveform.
- the second rotational stimulus uses a higher torque motor and thus its use would currently be limited to facilities that have chairs with higher torque motors.
- the second rotational stimulus produces a longer period of inhibition, which allows for a longer evaluation period, and better opportunity to assess the function of each ear. Therefore, the second rotational stimulus may provide better indication of asymmetry.
- the second type of rotational stimulus is a pulse-step-sine, or PSS stimulus which includes a pulse, step, and sinusoidal waveforms, as shown in FIGS. 3A-3B .
- the bias component includes 2 parts: a short-duration acceleration pulse waveform, followed by a lower amplitude, longer duration acceleration step waveform.
- the probe component, or probe has a sinusoidal waveform.
- the probe waveform is added to the acceleration step portion of the bias component.
- the bias component includes a short-duration, acceleration pulse (about 400°/s 2 amplitude lasting about 1 s), followed by a lower amplitude, longer duration acceleration step (about 30°/s 2 amplitude lasting about 4 s).
- the probe component has a sinusoidal waveform (about 1 Hz with 20°/s peak velocity), which is added to the acceleration step portion of the bias component.
- a set of computer algorithms that are used to analyze the eye movements evoked by embodiments of the rotational stimuli.
- the principle of the analysis algorithms is that responses to the bias and probe components of the stimulus are isolated from one another and separately analyzed. Both of these bias and probe component analyses provide quantitative measures relevant to the determination of vestibular function in each ear.
- the rotational stimuli are designed to take advantage of the 3D arrangement of the three pairs of semicircular canals in the two ears, and the physiological properties of the 8th nerve afferents innervating each semicircular canal.
- Each canal can be considered to be a fluid filled ring lying in a plane with a specific orientation in the head, and with respect to the other canals.
- Each canal plane can be defined by a vector perpendicular to that plane.
- afferent nerves innervating a canal will generate a response that depends on both the magnitude of the rotation and the orientation of the head rotation vector relative to the canal vector.
- each canal responds only to the component of the head rotation vector aligned with the canal vector (i.e. the projection of the head rotation vector onto the canal vector).
- the canals in opposite ears are pair-wise oriented and they operate in a push-pull manner.
- the horizontal canals in the two ears lie in approximately the same, roughly horizontal plane.
- a head rotation towards the left causes an increase in discharge rates of all left canal afferents, and a decrease in discharge rates of all right canal afferents.
- a head rotation to the right produces the opposite result.
- the remaining two pairs of vertical canals (left anterior-right posterior, and right anterior-left posterior) are oriented approximately perpendicular to the horizontal canal pair, perpendicular to one another, and about 45° with respect to the midline.
- a stimulus is provided to take advantage of peripheral nonlinearities.
- Push-pull action of semicircular canal pairs results in maximal excitation of afferents in one canal and maximal inhibition of the canal afferents in the opposite ear.
- Canal afferents are more easily driven to cutoff (zero discharge rate) than to saturation (maximal discharge rate). Once afferents are driven to cutoff, they cannot encode a change in rotational motion.
- a three-component stimulus includes a PSS stimulus.
- a two-sine stimulus includes two sinusoidal components.
- the first component is the low frequency bias component and the second component as the high frequency probe component.
- the purpose of the bias component is to generate a large cyclic shift in the discharge rate of afferents innervating one canal pair using a low frequency (for example 0.1 Hz), higher amplitude rotation.
- a high amplitude, low frequency bias component is used to drive the activity in one canal to zero during a portion of each stimulus cycle.
- a 0.1 Hz sinusoidal rotation with a 150-250°/s peak velocity is taken to be of sufficient magnitude to drive horizontal semicircular canal activity to zero.
- the probe component is added to the bias component to provide the 2-sine stimulus.
- the purpose of the probe component is to test the sensitivity of the system during different portions of the 10 s period of the bias component using a high frequency, low amplitude sinusoid (1 Hz, 20°/s in an embodiment).
- a low amplitude, high frequency probe component is used to test the ability of the vestibular system to encode this probe component throughout the entire stimulus cycle. If canal function is absent in one ear, and the bias component rotation drives the canal activity of the remaining good ear to zero, then motion due to probe component will not be encoded and VOR eye movements related to the probe component will be absent.
- the probe component response will be reduced or absent during the bias component rotation toward the dysfunctional ear.
- Use of a 1 Hz, 20°/s sinusoidal probe motion provides a good compromise between adequate signal-to-noise and motor torque requirements.
- FIGS. 4A-C shows a schematic response to a 2-sine stimulus 400 ( FIG. 4A ) in a normal subject 410 ( FIG. 4B ) and in a right side unilateral loss subject 420 ( FIG. 4C ).
- VOR eye movements do not show modulation related to the high frequency probe component during rotations toward the dysfunctional ear in the unilateral loss subject. If the amplitude of the 0.1 Hz component is adequately large, some proportion of afferents in the right and left horizontal semicircular canals (RHC and LHC) will be silenced during a portion of the 10 second (10 s) cycle period.
- the RHC encodes no motion at all, it is expected that in this embodiment the 1 Hz modulatory effects of the probe component will be reduced or absent in the VOR eye movements during rotations toward the defective ear.
- the LHC neural activity is able to encode both the bias and probe components, and the VOR eye movements will contain a 1 Hz component corresponding to the probe component of the stimulus.
- the 1 Hz modulation would only be completely absent if activity in all LHC afferents contributing to the VOR were driven to silence. In practice, it may not be necessary or practical to silence all activity in order to develop a useful clinical test stimulus.
- the magnitude of the rotational stimulus required to drive the discharge rate of afferents to zero depends upon both the resting discharge rate and sensitivity of the canal afferents. These values are unknown in humans, but are known in some animal models.
- the mean resting discharge rate is about 90 spikes/s (s.d. 36) and the mean sensitivity to a rotational acceleration is 2.24 spikes-s ⁇ 1 /deg-s ⁇ 2 (range 0.5-4).
- the mean sensitivity to a rotational acceleration is 2.24 spikes-s ⁇ 1 /deg-s ⁇ 2 (range 0.5-4).
- Embodiments for methods and apparatus include rotation test procedures that overcome poor performance of conventional rotation testing in identifying unilateral vestibular dysfunction, but maintains all of the advantages of conventional rotation testing (non-provocative/non-nauseogenic and good test-retest reliability), and therefore, can address the four diagnostic questions described in the previous paragraph.
- various embodiments take advantage of existing low torque rotation test devices already found in most major medical centers. This permits rapid adoption of embodiments of this new test into clinical practice.
- Such embodiments for a rotation test procedure would effectively eliminate the need for caloric testing, simplify the selection of appropriate diagnostic tests, and eliminate redundant testing.
- the routine application of various embodiments of this test would result in improved diagnosis of unilateral vestibular dysfunction, and therefore, would foster improved treatment of patients with chronic disabilities that are sometimes associated with unilateral vestibular dysfunction.
- Discussion of results to studies consists of six parts.
- First is a computer simulation illustrating the expected VOR responses to the 2-sine rotational stimulus described in the previous section with respect to FIGS. 4A-4C .
- Second is a demonstration that experimental VOR responses recorded using a 2-sine rotational stimulus are similar to the predicted simulation results.
- Third is a description of an embodiment of a data analysis procedure for parameterizing responses to the probe component of the 2-sine stimulus.
- Fourth are results from the probe component parameterization procedure applied to experimental data from 3 normal and 4 unilateral loss subjects.
- Fifth is a description of an embodiment of a procedure for analyzing the bias component response and the results of applying this procedure to normal and unilateral loss subject data.
- Sixth is a discussion of the applicability of the proposed stimulus and analysis procedures to subjects with partial unilateral loss of vestibular function.
- FIG. 5 shows a block diagram used to simulate VOR responses (implemented in Simulink, The MathWorks, Natick Mass.). Only the right and left horizontal canal pair is represented.
- the simulation 500 provides VOR response 510 to stimulus 520 and includes 3 weighted afferent channels in each ear. The weighting provides a rough approximation to a distribution of afferent properties.
- the input is considered to be head rotational velocity.
- each channel includes a high pass filter with a 5 s time constant representing the dynamics properties of the canals.
- each afferent channel is assumed to have the same resting discharge rate of 90 spikes/s, but the sensitivity of each channel varies.
- the afferent channel with the largest weight of 0.5 is assigned an acceleration sensitivity of 2.24 spikes-s ⁇ 1 /deg-s ⁇ 2 .
- the other two channels, each with a weight of 0.25, are assigned sensitivities of 4 spikes-s ⁇ 1 /deg-s ⁇ 2 and 1 spike-s ⁇ 1 /deg-s ⁇ 2 , respectively.
- the model parameters are based on physiological measures (for example afferent sensitivities and the mean discharge rate) in the squirrel monkey (See references 16 and 24 above). A saturation nonlinearity prevents afferent discharges from falling below zero spikes/s.
- the stimulus is assumed to be rotational velocity.
- All channels pass through a nonlinear element that clips the discharge rate at zero for rotational motions that inhibit the afferents, but permits unlimited positive discharge rates.
- the VOR eye velocity response output is assumed to be proportional to the discharge rate of all right side afferents minus all left side afferents. To simulate a complete unilateral absence of vestibular function, the sensitivities of all afferents in one ear are set to zero.
- FIGS. 6A-6F shows the simulated VOR output (simulated VOR slow phase eye velocity response to a 2-sine stimulus in normal and left unilateral loss subjects) for the embodiment of the simulation of FIG. 5 .
- the bias component stimulus amplitude increased from 0 to 250°/s, but the probe stimulus amplitude remained constant.
- This simulated VOR output illustrates that no response asymmetry is grossly evident when the bias component amplitude was below about 100°/s. However, at higher bias component amplitudes, the response became increasingly asymmetric with reduced responses during rotations toward the absent ear.
- FIGS. 7A-7E show the portion of the simulated VOR eye velocity related to the high frequency probe component for the embodiment of the simulation of FIG. 5 .
- the high frequency VOR component was present throughout the 10 s period only when the bias component amplitude was 100°/s and below. But when the bias component amplitude was increased (150 to 250°/s), the probe component VOR was reduced during the portion of the bias component cycle corresponding to rotation towards the dysfunctional ear.
- the unilateral loss subjects included subject UL 1 , a 66 year old female who had a left side acoustic neuroma removed by a trans-labyrinthine surgical approach 3 years prior to testing, subject UL 2 , a 46 year old male with a 3 cm left side acoustic neuroma treated with a “gamma knife” radiation procedure 31 ⁇ 2 years prior to testing, subject UL 3 , a 27 year old male with right side absent vestibular function, as determined by caloric testing (Meningitis contracted during infancy is believed to be the cause of this right side loss), and subject UL 4 , a 47 year old female with a right labyrinthectomy performed 3 months prior to testing as treatment for Meniere's disease.
- All 4 unilateral loss subjects were well compensated as judged by their responses to conventional rotation tests (0.05, 0.2, and 0.8 Hz sinusoidal rotations with 60°/s peak velocity).
- Three of the four unilateral loss subjects showed abnormally advanced VOR response phase at 0.05 Hz (>20° phase lead).
- the fourth subject's (UL 1 ) phase was advanced relative to center normal, but was not outside of the established lab normal range.
- Phase advance is typically the only abnormal finding with conventional rotation testing in well compensated unilateral loss subjects. However, this abnormal finding does not provide any information regarding the side of the vestibular loss.
- test stimuli were well tolerated by the three normal and four unilateral loss subjects tested to date. No discomfort, disorientation, or motion sickness symptoms were reported throughout the test sessions which lasted about 2 hours. Following testing, there were no complaints of imbalance, and none of the subjects had difficulty walking or maintaining stance.
- FIGS. 8A-8F show the VOR slow phase horizontal eye velocity data recorded from one representative normal subject and the four unilateral vestibular loss subjects in this embodiment to a 2-sine simulation. These experimental responses are qualitatively similar to the simulation results ( FIGS. 6A-6F ).
- the normal subject showed symmetric VOR responses for rightward and leftward directed rotations that increased with increasing stimulus velocity.
- the response to the 1 Hz probe component for the normal subject was evident throughout the entire 10 s cycle of the bias component.
- Three of the four unilateral loss subjects (UL 1 , UL 2 , UL 4 ) showed reasonably symmetric responses when the bias component amplitude was 50°/s. Responses of the unilateral loss subjects resembled normal subject responses for bias component amplitudes of 0 and 50°/s. Subject UL 3 began to show some asymmetry even at a 50°/s bias component amplitude. In contrast to the normal subject responses, the asymmetry increased for all UL subjects as the bias component amplitude increased. This asymmetry was always such that the VOR eye velocity was reduced during rotation toward the dysfunctional ear.
- the VOR response to the probe component was not uniform throughout the 10 s bias component cycle, but was reduced during the portion of the bias component cycle when the subject was rotating towards the dysfunctional ear.
- Responses of the unilateral loss subjects showed saturation during rotation toward the absent ear at higher bias component amplitudes.
- FIGS. 9A-9F show the modulation of the VOR response to the probe component with the VOR slow phase velocity data filtered using a 0.5 to 5 Hz bandpass filter.
- VOR modulation of the probe response is diminished during rotations towards the dysfunctional ear.
- the unilateral loss subjects showed a systematic modulation of the VOR probe component amplitude over the 10 s bias component cycle. This modulation increased with increasing bias component amplitude.
- the normal subject did not show a systematic increase in probe component modulation with increasing bias component amplitude.
- the normal subject did not show the “double modulation” evident in the simulated VOR responses at the 200 and 250°/s bias component velocities, indicating that the simple model in FIG. 5 does not fully capture actual VOR behavior. All 3 normal subjects tested had results similar to those shown in FIGS. 9A-9F .
- FIG. 10A depicts a block diagram of an embodiment for analysis of a probe component.
- the flow diagram for the embodiment of the method shown in FIG. 10A is used to characterize the response to the probe component portion of the VOR.
- Example data in FIG. 10B are from subject UL 1 of FIGS. 9A-9F , with a left side unilateral loss, tested with a 200°/s bias component amplitude.
- the analysis includes both standard methodology, typically applied to VOR analysis, and novel methodology used to separate and analyze the probe response.
- a video analysis is performed, at 1010 , to acquire pupil center coordinates (x c , y c ) and a calibration is applied, at 1020 , to provide a horizontal eye position and a vertical eye position as a function of the pupil center coordinates (x c , y c ).
- the standard portion of the analysis includes calculation of eye velocity from eye position data, at 1030 , and separation of the slow and fast phases of nystagmus in order to obtain slow phase eye velocity, at 1040 .
- the novel aspects of the analysis include bandpass filtering to isolate the response to the probe component of the stimulus, at 1050 , and parameterization of the probe response, at 1070 . As shown in FIG.
- the filtered signal may be averaged, at 1060 , such as averaging over five 0.1 Hz cycles.
- the parameterization of the probe response uses a curve fit.
- the bandpass response is filtered over a number of cycles of the bias component. In an embodiment, the bandpass response is filtered over five 0.1 Hz cycles.
- the curve fit is performed using a constrained nonlinear optimization procedure “const” available in the Matlab Optimization Toolbox (the curve fit actually includes sine and cosine components from which the phase of the response relative to a cosine reference is calculated).
- This equation represents an amplitude modulation (AM) operation used to describe communication systems where a “carrier” waveform, represented here by the probe frequency ( ⁇ p ), is modulated by a lower frequency waveform using a multiplicative operation. In this case the lower frequency modulation occurs at the bias component frequency ( ⁇ b ).
- AM amplitude modulation
- Modulation factor, m is related to the severity of the unilateral asymmetry. Modulation phase, ⁇ b , indicates the side of unilateral loss.
- a p equals the 1 Hz probe component eye velocity amplitude and m equals the modulation factor representing the depth of modulation of the probe component eye velocity.
- FIG. 11 shows a plot of the modulation factor, m, for the three normal and four unilateral loss subjects of the experiment related to FIGS. 8A-8F and FIGS. 9A-9F as a function of the bias component stimulus amplitude. It can be seen that at lower bias component amplitudes, this parameter is unable to distinguish between normal and unilateral loss subjects. However, a clear separation is evident between subject UL 3 and normals at a bias component amplitude of 100°/s, and at 150°/s all four unilateral loss subjects are distinguishable from normals.
- the modulation parameter increases with increasing bias component amplitude. For subject UL 3 , the modulation parameter saturated at 1 (full modulation) for the 200°/s and 250°/s bias component amplitude.
- head orientation does affect the modulation factor in unilateral loss subjects. For example, for subject UL 4 tested with a bias component amplitude of 200°/s, m increased from 0.42 to 0.58 with a head tilt of 5° in a direction expected to bring the horizontal canals into better alignment. For subject UL 3 , also tested with a 200°/s bias component amplitude, m decreased from 1.0 to 0.81 when the head was reoriented by 5°. Therefore, head orientation appears to be an important factor influencing the sensitivity for various embodiments of the rotation test. The influence of head orientation can be systematically investigated using both modeling and experimental methods.
- FIG. 12 depicts a block diagram of an embodiment of a method to analyze the bias components of the 2-sine stimulus.
- the flow diagram for the embodiment of the method shown in FIG. 12 shows a procedure to analyze distortion in the bias component of the 2-sine stimulus.
- This method may be related to methods previously used to analyze VOR gain asymmetries.
- Example data in this figure are from subject UL 1 in FIGS. 8A-8F tested with a 200°/s bias component amplitude.
- slow phase eye velocity and stimulus velocity data are low pass filtered (in an embodiment, a low pass filter having a 0.5 Hz cutoff is used) to remove the probe component from both the stimulus and VOR response waveforms.
- the data is averaged over a number of cycles of the bias component.
- the data are then averaged over five 0.1 Hz cycles. In an embodiment, the data are then averaged over consecutive 10 s periods corresponding to the bias component cycle period.
- a discrete Fourier transform is used to estimate the phase of the stimulus and response waveforms at the bias component frequency.
- these waveforms are then time shifted so that they are aligned with a 180° phase shift between them (reflecting the compensatory nature of the VOR).
- a negatively sloped input-output function is obtained by plotting the eye velocity versus the stimulus velocity.
- FIG. 13 shows an embodiment for a input-output relationship generated according to the embodiment in FIG. 12 .
- deviations of this input-output function from a straight line indicate the presence of nonlinear system effects, in this case a saturation-type nonlinearity where the eye velocity is attenuated at higher velocities of rotations toward the ear with absent vestibular function.
- FIG. 14 shows the results of an embodiment of the input-output analysis of FIG. 12 for the four unilateral loss subjects and two normal subjects of FIGS. 8A-8F .
- All data in FIG. 14 are from a 2-sine stimulus with a 200°/s bias component amplitudes.
- rotations toward the dysfunctional ear produced a clear saturation-type nonlinearity compared to rotations toward the good ear.
- Consistent estimates of saturation amplitudes were obtained using 200 and 250°/s bias component stimuli, and often with 150°/s stimuli.
- the normal subjects had symmetric, although not entirely linear, input-output functions with no saturation.
- ⁇ ⁇ . ⁇ lp ′ ⁇ K ⁇ ( 1 - e - ⁇ ⁇ ⁇ ⁇ lp ′ ⁇ ) 1 + e - ⁇ ⁇ ⁇ ⁇ lp ′ ⁇ ( eqn . ⁇ 2 )
- ⁇ dot over ( ⁇ circumflex over ( ⁇ ) ⁇ ′ tp > is a fit to the low pass filtered bias component eye velocity
- ⁇ ′ lp > is the low pass filtered bias component stimulus velocity
- K and ⁇ are fit parameters related to the gain and saturation behavior of the input-output function.
- Parameter K is the saturation amplitude (°/s), ⁇ is the saturation rate, and ⁇ lp > is the phase aligned bias component of the stimulus velocity. Since the saturation function given by eqn. 2 is symmetric about the origin, separate fits of this functional form would be required for rotations toward the right and left.
- the probe component analysis essentially provides an independent comparison of VOR gain at the peak positive and negative excursions of the bias component stimulus. These two peak points in the bias component stimulus cycle are most likely to be influenced by the existence of asymmetric vestibular function.
- the input-output analysis of the bias component response is dominated by large portions of the input-output function which are likely to be symmetric even though a partial vestibular asymmetry exists.
- an embodiment using rotational stimulus appears to be effective in unambiguously identifying the side of lesion.
- additional work can be performed to determine optimal stimulus design, investigate factors that influence test sensitivity and reliability, and apply the various embodiments for the rotational test to a larger group of subjects with varying levels of vestibular dysfunction.
- the frequency components are shown in Table 1.
- the amplitude of the bias component is be varied from 50°/s to 250°/s in increments of 50°/s while the probe component remains fixed in frequency and amplitude.
- a pulse-step-sine (PSS) stimulus representing a form of 3-component rotational stimulus, can be applied to produce a large shift in the discharge rate of the primary afferents, followed by testing at the extremes of the discharge rate shift.
- the PSS stimulus provides a potentially optimal stimulus for rapidly displacing the afferent discharge rate, maintaining that discharge rate displacement at a constant level, and then testing the system.
- the PSS stimulus includes a short duration acceleration pulse followed by a lower amplitude, longer duration acceleration step.
- a sinusoidal probe component is added to the acceleration step. With proper selection of the acceleration pulse and step components, the afferent discharge rate will theoretically remain at a constant shifted level throughout the duration of the step phase of the stimulus. Therefore, the sinusoidal probe component can test the system while the afferent discharge rate is at a fixed displacement from the resting rate. This is in contrast to a 2-sine stimulus where the afferent discharge rate is continuously changing throughout the bias component cycle.
- FIGS. 16A-16C The stimulus acceleration, velocity, and simulated afferent discharge rate profiles for an example PSS stimulus are shown in FIGS. 16A-16C .
- the trace of FIG. 16A shows the rotational acceleration profile for the example PSS stimulus.
- the trace of FIG. 16B shows the velocity profile for the example PSS stimulus.
- the trace of FIG. 16C shows a simulated response to the PSS stimulus of an average squirrel monkey canal afferent. Variations in the pulse duration and amplitude determine the displacement of afferent discharge rates from resting levels.
- the acceleration step amplitude needed to maintain that displaced discharge rate is a function of the preceding pulse parameters and the time constant of the afferent nerve fibers. In an embodiment, a 5 s time constant is assumed in the stimulus design.
- a larger step acceleration is needed to maintain a fixed discharge rate.
- Several cycles of a sinusoidal motion are added to the step component to provide a probe stimulus to test the VOR system while the afferents remain at a displaced discharge rate.
- Table 2 shows a series of PSS stimuli with each subsequent stimulus in the series producing a larger afferent discharge displacement.
- the cycle length for each of these 4 stimuli is 10 s.
- the predicted displacement of afferent discharge is matched to the peak displacement predicted for the 0.1-1.0 Hz 2-sine stimuli with the 0.1 Hz bias component amplitude varying from 50°/s to 200°/s.
- PSS analysis involves measurement of the amplitude of the sine component for rotations to the right and left.
- An asymmetry measure (similar to the VOR gain asymmetry measure used to characterize conventional rotation test responses) calculated from these amplitude measurements would correspond approximately to the modulation factor, m, calculated from the probe analysis for the 2-sine stimuli. If the response to the sine component of the PSS stimulus is ignored (by low pass filtering), the remainder of the PSS slow phase velocity response should approximate a square wave (with rounded corners). A displacement of the mean value of this slow phase velocity waveform (mean calculated over an integer number of cycles) away from zero would be indicative of an asymmetry of this response, and would correspond approximately to the input-output analysis of the bias component of the 2-sine stimulus.
- response parameters e.g. m and parameters summarizing the asymmetry of input-output functions
- response parameters may be plotted as a function of the bias component amplitude, as in FIG. 11 plots of m versus bias component amplitude.
- Similar plots may be made for measures derived from the PSS stimuli, with PSS response asymmetry measures plotted versus an equivalent to the bias component amplitude (specifically, the peak velocity amplitude of the fundamental frequency component of the PSS stimulus).
- the mean, standard deviation, and range of response parameters for the normal and unilateral loss groups may be computed for each of the four different test series given in Table 1 and the PSS series in Table 2. Assuming approximately normal distributions, but not necessarily equal variances between the groups, a “threshold bias amplitude,” defined as the point where the separation between results from the normal and unilateral loss group have a maximum 5% misclassification of normal subjects as abnormal and abnormal subjects as normal (95% specificity and 95% sensitivity), can be estimated. Test series may be compared by their threshold bias amplitudes. The series with the lowest threshold bias amplitude will be presumed to be the most sensitive test for detecting a unilateral vestibular asymmetry.
- the pulse component provides a rotational acceleration pulse that drives the activity in one canal toward zero during a portion of each stimulus cycle.
- An acceleration step maintains the canal nerve activity at a zero discharge rate throughout the duration of the step component.
- the sine component is a low amplitude, high frequency sine waveform that is added to the step portion of the stimulus.
- the sine component tests the ability of the vestibular system to encode this component throughout the step portion of the stimulus. If canal function is absent in one ear, and the pulse-step components drive the canal activity of the remaining good ear to zero, then motion due to the sine component will not be encoded and VOR eye movements related to the sine component will be absent.
- the sine component response will be reduced or absent during pulse-step component rotation toward the dysfunctional ear.
- Use of a 1 Hz, 20°/s sinusoidal component motion provides a good compromise between adequate signal-to-noise and motor torque requirements.
- An experimental application using four different PSS stimuli included ten subjects with normal vestibular function, five subjects with complete unilateral vestibular loss (three left and two right), and one subject with asymmetric bilateral vestibular loss with the right side less than the left side.
- the testing of the subjects included rotation about earth-vertical axis in the dark and the head orientated 20° nose down from Reid's plane to place horizontal canal plane perpendicular to rotation axis.
- the parameters for the four PSS rotational stimuli were a pulse component having a 400°/s 2 amplitude with durations ranging from 0.25 to 1 s, a step component having amplitudes ranging from 10 to 38°/s 2 with durations ranging from 4 to 4.8 s, and a sine component with either 3.5 or 4.5 cycles of approximately 1 Hz sinusoid with 20°/s amplitude (Table 2).
- the responses were symmetric for leftward and rightward directed rotations. Response to the sine component was evident throughout both rightward and leftward step components.
- the responses resembled normal subject response for lowest amplitude PSS stimulus.
- the responses became increasingly asymmetric with increasing PSS amplitude with loss of the sine response during rotation toward the absent ear.
- the responses became increasingly asymmetric with increasing PSS amplitude with step component response showing the primary asymmetry.
- Measures for PSS rotational stimulus are generated from slow/fast phase separation of eye velocity data and averaging over a number of cycles of the bias component of the stimulus, in which the response to the sinusoidal stimulus is isolated from the response to the pulse-step component.
- PSS Bias (or Step) Component Measures.
- the mean response slope parameter is a measure that is related to a measure called the “VOR time constant” that is often obtained from responses to a conventional clinical rotation test.
- the value of a patient's VOR time constant can provide information about vestibular dysfunction. Specifically, a VOR time constant that is less than 5 seconds is typically associated with a bilateral loss of vestibular function. A time constant of 5 to 6 seconds is often associated with a unilateral vestibular loss. Finally, a time constant greater that about 8 seconds suggests normal vestibular function.
- the relation between the VOR time constant and the mean response slope is that subjects with a VOR time constant less than 5 seconds will have a slope with value less than zero.
- the mean response slope will be equal to zero if the VOR time constant is 5 seconds, and the mean response slope will be greater than zero if the VOR time constant is greater than 5 seconds. Therefore, this slope parameter provides equivalent information to that provided by an important parameter measured using conventional rotational stimuli. Other parameters, such as the step asymmetry and measures related to the probe component, go beyond what is available from conventional rotation tests and facilitate the identification of the side of the vestibular loss.
- the parameter called “sine component asymmetry” gives a comparison of the difference in VOR probe-component gains for rotations that evoke leftward eye movements versus rotations that evoke rightward eye movements.
- FIGS. 17A-17B The measures shown in FIGS. 17A-17B were used to determine response parameter variation with PSS amplitude for the subjects of the PSS stimulus test.
- the step component asymmetry and sine component symmetry measures were tightly distributed about zero.
- the step component slopes were greater than 4°/s 2 for most normal subjects, indicating that their VOR time constants were greater than 5 s. Exceptions were on a normal subject with poor quality data (VOR suppression) and one overly tested normal subject.
- the step component asymmetry and sine component asymmetry measures generally indicated the side of vestibular dysfunction and were distinguishable from normal subject results even with the smallest amplitude PSS stimulus. Differences between normal and unilateral loss subjects increased with increasing PSS amplitude. The step component slopes were closer to zero than for most normal subjects, indicating that unilateral loss subjects have smaller VOR time constants compared to normal subjects.
- step component and sine component asymmetry measures indicated the side of greater vestibular loss, but the step component asymmetry measure showed larger deviation from normal.
- the step component slope measure was negative and lower than either normal or unilateral loss subjects, consistent with a reduced VOR time constant ( ⁇ 5 s).
- the third VOR response measure provided information about the VOR time constant which is known to be reduced in unilateral vestibular loss subjects, and further reduced in bilateral vestibular loss subjects.
- the large amplitude rotational motions required for the PSS stimulus were well tolerated by all subjects.
- the large amplitude rotations theoretically enhance ability to distinguish between normal and unilateral loss subjects, but also result in decreased accuracy of VOR response estimates due to the presence of high frequency nystagmus. It is likely that PSS stimuli with peak velocities in the range of 150-225°/s will provide for optimal identification of asymmetric vestibular function.
- the same rotational stimuli (2-sine stimulus and PSS stimulus) that are used for the Bias-Probe rotation test performed in the dark can also be used with a visual fixation light.
- This version of the Bias-Probe rotation test is performed using a single fixation light placed in front of the subject, where this light rotates with the subject. The room in which this test is performed is otherwise dark.
- the purpose of using a visual fixation light is to partially suppress the eye movements evoked by the vestibulo-ocular reflex (VOR).
- VOR vestibulo-ocular reflex
- the advantage of partially suppressing the VOR is that lower velocity eye movements are obtained that are often more consistent over time (compared to the VOR eye movements obtained in the dark).
- VOR eye movements are “consistent” when they show a continuous relationship to the rotational stimulus velocity. More consistent eye movements provide more reliable measures that are better able to distinguish between normal and abnormal vestibular function.
- Example eye movement data from two normal subjects are shown in FIGS. 18A-18B .
- One of these normal subjects had “good” nystagmus (eye movements) during a PSS stimulus performed in the dark, and the other subject (lower trace of FIG. 18A ) had “bad” nystagmus.
- the distinguishing feature of good versus bad nystagmus is the consistency of the eye movements over time and the relationship of the eye movements to the stimulus rotation.
- the nystagmus was partially suppressed in the subject with good nystagmus, but remained consistent over time (middle trace of FIG. 18B ).
- FIGS. 19A-19B Similar results are shown in FIGS. 19A-19B for two patients with a unilateral vestibular loss.
- the patient with a right unilateral loss (middle traces of FIGS. 19A-19B ) had consistent nystagmus on PSS tests performed in both the dark and with the fixation light.
- the patient with a left unilateral loss had inconsistent nystagmus in the dark (lower trace of FIG. 19A ), but consistent nystagmus when tested with the fixation light (lower trace of FIG. 19B ).
- FIGS. 19A-19B showing the VOR with fixation for the unilateral vestibular loss patients, there is a large asymmetry between eye movements evoked during rotation to the left (positive PSS stimulus velocity) versus rotation to the right (negative PSS stimulus velocity).
- the eye movements evoked by Bias-Probe rotation tests with fixation can be analyzed using the same principles and methods developed for the analysis of Bias-Probe rotation tests performed in the dark. Specifically, the slow-phase velocity eye movement responses to the bias component and the probe component can be separated from one another, and curve fits performed to estimate parameters that characterize vestibular function and response symmetry. This applies for both the 2-sine and PSS rotational stimuli.
- Unilateral vestibular loss patients show two types of VOR fixation asymmetries not present in normal subjects.
- First, unilateral vestibular loss patients are better able to visually suppress VOR eye movements during bias component rotation toward the dysfunctional ear than during rotation toward the intact ear because the total change in semicircular canal afferent nerve discharge rate is lower during rotation toward the dysfunctional ear than during rotation toward the intact ear.
- Second, VOR responses caused by the probe component are absent or reduced during the portion of the rotation stimulus when the patient is rotating toward the dysfunctional ear because canal afferent activity is silenced in the intact ear and the dysfunctional ear is unable to encode the probe component stimulus.
- FIGS. 20A-20B Response parameters obtained from fixation test performed on subject with normal vestibular function and patients with unilateral vestibular loss subjects are shown in FIGS. 20A-20B .
- FIG. 20A shows a “step component asymmetry” measure that refers to a measure obtained from the response to the bias component of the PSS stimulus.
- FIG. 20B shows a “sine component asymmetry” measure that refers to a measure obtained from the response to the probe component of the PSS stimulus.
- sine component asymmetry measure did not provide a good separation between normal and unilateral loss are attributable to unilateral loss subjects who were able to nearly completely suppress their VOR using the fixation light.
- the inadequacy of the existing tests can be partially resolved by performing multiple tests on the same patient, using a combination of the existing tests.
- some patients due to their physical limitations cannot tolerate certain of the tests (e.g. head pulse test), and in any case, multiple tests increase the cost of obtaining a diagnosis.
- head pulse test e.g. head pulse test
- multiple tests increase the cost of obtaining a diagnosis.
- physicians usually settle for a single test, and the referred test has been the caloric test with all of its attendant limitations.
- Various embodiments for methods and apparatus according to the teachings of the present invention provide rotation test and analysis that overcome many of the poor performance features found in existing tests and provides a new diagnostic tool to identify normal and abnormal vestibular function, to localize an abnormality to a particular ear, and to evaluate the severity of the abnormality. This information is critical for the physician s delivery of therapies targeted to the patient's specific vestibular condition.
- the use of rotational motions with amplitudes larger than typically used in conventional clinical rotation tests can reveal asymmetric VOR responses in subjects with unilateral vestibular loss.
- This VOR asymmetry can be reliably measured and used to identify the dysfunctional ear.
- the VOR asymmetry is revealed as a saturation nonlinearity during rotations towards the absent ear.
- Rotational motions greater than about 150°/s are sufficient to quantify the VOR saturation amplitude in unilateral loss subjects.
- the inclusion of a high frequency, low amplitude probe component in the rotational stimulus provides a second measure that identifies the presence and side of abnormal vestibular functions.
- the larger amplitude rotational motions required for the two-sine and PSS stimuli were well tolerated by tested subjects.
- Optimal test sensitivity may be achieved when the head is oriented with the canal planes perpendicular to the rotation axis. However, it may be difficult to achieve this optimal orientation in each subject since external landmarks are not tightly correlated with canal orientation. Large amplitude rotations enhanced the ability to distinguish between normal and unilateral loss subjects, but also resulted in decreased accuracy of VOR response estimates due to the presence of high frequency nystagmus. It is likely that a bias component peak velocity in the range of 150-200°/s will provide for optimal identification of asymmetric vestibular function.
Abstract
Description
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<{dot over ({circumflex over (θ)}bp >=A p(1+m cos(ωb t+φ b))cos(ωp t+φ p) (eqn. 1a)
<{dot over ({circumflex over (θ)}bp >=A p(1+M(t))cos(ωp t+φ p) (eqn. 1b)
In an embodiment, the bandpass response is filtered over a number of cycles of the bias component. In an embodiment, the bandpass response is filtered over five 0.1 Hz cycles.
M(t)=m cos(ωb t+φ b)
with m being a modulation factor, which can vary from 0 to 1, representing the depth of modulation of the probe frequency, the probe component phase, φp, and the phase of the modulation waveform, φb. Modulation factor, m, is related to the severity of the unilateral asymmetry. Modulation phase, φb, indicates the side of unilateral loss. In an embodiment, Ap equals the 1 Hz probe component eye velocity amplitude and m equals the modulation factor representing the depth of modulation of the probe component eye velocity.
where <{dot over ({circumflex over (θ)}′tp> is a fit to the low pass filtered bias component eye velocity, <ω′lp> is the low pass filtered bias component stimulus velocity, and K and β are fit parameters related to the gain and saturation behavior of the input-output function. Parameter K is the saturation amplitude (°/s), β is the saturation rate, and <ωlp> is the phase aligned bias component of the stimulus velocity. Since the saturation function given by eqn. 2 is symmetric about the origin, separate fits of this functional form would be required for rotations toward the right and left.
TABLE 1 |
2-sine stimulus frequency combinations |
Probe Component |
Stimulus # | Bias Component Frequency (Hz) | Freq (Hz) | Amp (°/s) |
1 | 0.025 | 1 | 20 |
2 | 0.05 | 1 | 20 |
3 | 0.1 | 1 | 20 |
4 | 0.1 | 2 | 10 |
ΔR=S aff A pulse(1—e−1p/2τ) eqn. 3
where Saff is the acceleration sensitivity of the afferent, Apulse is the acceleration pulse amplitude of the PSS stimulus, tp is acceleration pulse duration, and τ is the afferent time constant.
TABLE 2 |
PSS stimuli |
Pulse | Step | Sine | |
Component | Component | Component |
PSS | Amp | Duration | Amp | Duration | Freq | Amp | |
Stimulus | (°/s2) | (s) | (°/s2) | (s) | (Hz) | (°/s2) | Cycles |
1 | 400 | 0.25 | 9.9 | 4.75 | 0.95 | 20 | 4.5 |
2 | 400 | 0.51 | 19.9 | 4.49 | 1.0 | 20 | 4.5 |
3 | 400 | 0.78 | 30.0 | 4.22 | 0.83 | 20 | 3.5 |
4 | 400 | 1.05 | 39.9 | 3.95 | 0.89 | 20 | 3.5 |
-
- ASL=average stimulus velocity (units °/s) during the leftward-moving step portion of the PSS stimulus. The subscript “S” stands for stimulus and the subscript “L” stands for leftward.
- ASR=average stimulus velocity (units °/s) during the rightward-moving step portion of the PSS stimulus.
- ARL=average VOR slow phase eye velocity (units °/s) during the leftward-moving step portion of the PSS stimulus. The subscript “R” stands for response and the subscript “L” stands for leftward and refers to the fact that this eye velocity is the average value measured during the leftward stimulus motion. The eye velocity is typically directed toward the right during a leftward stimulus motion.
- ARR=average VOR slow phase eye velocity (units °/s) during the rightward-moving step portion of the PSS stimulus. The first subscript “R” stands for response and the second subscript “R” stands for rightward and refers to the fact that this eye velocity is the average value measured during the rightward stimulus motion. The eye velocity is typically directed toward the left during a rightward stimulus motion.
The mean response slope parameter is given by the equation:
Mean Response Slope=(S RR −S RL)/2,
where SRR is the rate-of-change (i.e. a slope) of slow phase eye velocity (units °/s2) measured during the rightward-moving step portion of the PSS stimulus. The first subscript “R” stands for response and the second subscript “R” stands for rightward and refers to the fact that this eye velocity is the average value measured during the rightward stimulus motion. SRL is the rate-of-change (i.e. a slope) of slow phase eye velocity (units °/s2) measured during the leftward-moving step portion of the PSS stimulus. The subscript “R” stands for response and the subscript “L” stands for leftward and refers to the fact that this eye velocity is the average value measured during the leftward stimulus motion.
VOR L =R L /S L
VOR R =R R /S R,
where:
-
- RL=peak slow phase eye velocity of the response (units °/s) to the probe (or sine) component of the PSS stimulus during the portion of the PSS when the step component is leftward-moving.
- RR=peak slow phase eye velocity of the response to the probe (or sine) component of the PSS stimulus during the portion of the PSS when the step component is rightward-moving.
- SL=peak amplitude the probe (or sine) component (units °/s) of the PSS stimulus during the portion of the PSS when the step component is leftward-moving.
- SR=peak amplitude the probe (or sine) component (units °/s) of the PSS stimulus during the portion of the PSS when the step component is rightward-moving.
Nominally, SR=SL. The peak slow phase eye velocity values, RL and RR, are derived from a curve fit of a sinusoidal function to the average probe-component eye velocity. The expectation is that the sine component asymmetry parameter will have a positive value for a patient with right side vestibular loss, a negative value for a patient with left side vestibular loss, and a value close to zero for subjects with normal vestibular function.
Claims (85)
<{dot over ({circumflex over (θ)}bp >=A p(1+m cos(ωb t+φ b))cos(ωp t+φ p).
((ARL/ASL)−(ARR/ASR))/((ARL/ASL)+(ARR/ASR)).
VOR L =R L /S L
VOR R =R R /S R.
(VORL−VORR)/(VORL+VORR).
<{dot over ({circumflex over (θ)}bp >=A p(1+m cos(ωb +φ b))cos(ωp +t+φ p).
((ARL/ASL)−(ARR/ASR))/((ARL/ASL)+(ARR/ASR)).
VOR L =R L /S L
VOR R =R R /S R.
(VORL−VORR)/(VORL+VORR).
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